1. Field of the Invention
This invention relates to superficially porous metal oxide particles and to methods for making them, as well as to separation devices containing superficially porous particles.
2. Background Art
Superficially porous metal oxides, particularly silica particles, are used in chromatography columns to separate mixed substances from one another, as well as in other applications. Such particles consist of a nonporous core with an outer porous shell. High performance liquid chromatography (“HPLC”) columns containing superficially porous silica particles have short mass transfer distances, resulting in fast mass transfer; and thus fast separation.
U.S. Patent Publication No. 2007/0189944, by Kirkland et al., describes three conventional methods to prepare superficially porous silica particles. The first method is a spray-drying method, wherein solid silica particles or cores are mixed with a silica sol, and the mixture is sprayed under high pressure through a nozzle into a drying tower at high temperature (e.g., 200° C.). Unfortunately, the particles made this way often are incompletely or un-homogeneously coated. Such particles invariably also contain significant concentrations of unwanted totally porous particles of similar size, which come from the sol. Elutriation-fractionation of this product often fails to remove the totally porous contaminating particles, making the spray-drying approach less than optimal for producing the desired particles. In addition, the spray drying method can only make particle sizes larger than 5 μm, most in 30-100 μm range, and such particles have broad particle size distributions.
A second conventional method is “multilayer technology,” in which solid silica cores are repeatedly coated with layers of colloidal particles by alternating layers of oppositely charged nanoparticles and polymers containing amino-functional groups until the particles reach the desired sizes. Such methods are described in U.S. Pat. No. 3,505,785, issued to Kirkland, and U.S. Patent Publication No. 2007/0189944, by Kirkland et al. Even at its best, the process is labor intensive, and very difficult to practice. When such a method is applied on small cores with size less than 2 μm, the final particle surface tends to become less spherical and rougher. The process generates a lot of different types of aggregated particles, resulting in loss of yield of the desired particles.
A third conventional method involves coacervation. In this method, solid silica spheres are suspended in silica sol under acidic conditions. A coacervate of urea-formaldehyde polymer and ultra-pure silica sol is thus formed and becomes coated on the solid spheres (see, e.g., Kirkland, Journal of Chromatography A, 890 (2000) 3-13). The urea-formaldehyde polymer is then removed by burning at 540° C., and the particles are then strengthened by sintering at an elevated temperature. This procedure is much simpler and more practical compared to the multilayer technology described above. However, the coacervation method has its drawbacks. One is that some of the solid particles often are not coated, leaving non porous particles in the finished product. Another is that much smaller totally porous particles are formed along with the coated and uncoated particles. This latter drawback necessitates further classification of totally porous particles and superficially porous particles.
Thus, conventional methods of preparing superficially porous silica particles all use silica nanoparticles as the building blocks, on which an outer porous shell is added. As a result, the porous shell has randomly distributed pores with wide pore size distribution. Moreover, the resulting rough external particle surfaces limit the performance of columns containing such particles at high flow rates by generating an unusually high film mass transfer resistance. Rough surfaces also limit the packing density because of increased friction forces among particles during the packing process (Gritti, et al., J. Chromatogr. A, 1166 (2007) 30-46).
Micelle-templated silica synthesis of totally porous silica particles through pseudomorphic transformation has been reported (see e.g. Martin, Angew. Chem. Int. Ed., 41 (2002) 2590). In contrast with the earlier techniques, where pores are randomly distributed, micelle-templated synthesis produces a more ordered pore framework involving preformed micellar structures via a liquid crystal templating mechanism (see, Kresge, Nature, 359, 710 and U.S. Pat. No. 5,057,296).
Pseudomorphism is a term used by mineralogists to describe phase transformation that does not change the shape of a material. Thus, the pseudomorphic synthesis mentioned here, assisted by a surfactant, for totally porous pre-shaped silica particles reportedly forms a highly ordered narrow mesopore size distribution, high surface area and pore volume without changing the initial shape of silica particles. The high specific surface area, high pore volume, and adjustable pore size should improve the retention capacity and molecular selectivity, as well as provide an overall improvement in mass transfer between the stationary and mobile phase.
Lefevre reportedly synthesized 10 μm totally porous silica particles with pore diameters ranging from 7 to 9 nm, specific surface areas of 900 m2/g, and pore volumes of 1.5 ml/g (see, “Synthesis of Large-Pore Mesostructured Micelle-Templated Silicas as Discrete Spheres,” Chem. Mater., 2005, 17, 601-607). The synthesis started with totally porous silica particles as a starting material in a sealed autoclave in a basic solution at above the boiling point of water from several hours to days, where the solution contained a micelle agent such as cetyltrimethylammonium bromide and a swelling agent such as trimethyl benzene. After the reaction, the micelle and swelling agents were removed by burning them off. However, they reported that large pore, totally porous silica particles with a particle size smaller than 8 μM cannot be made by this method due to particle aggregation. They also reported that particle explosion can occur if the pore volume of the totally porous silica starting material is too low. Thus, they start with totally porous particles (pore volume larger than 0.7 cm3/g) and produce totally porous particles with a more ordered pore structure and higher surface area than the starting material.
Hybrid (i.e., covalent bonding between the organic and inorganic components within the material) totally porous silica particles have become popular as an HPLC packing material for HPLC columns because of their stability at high pH as well as their high physical strength. One method to prepare hybrid totally porous silica particles uses emulsion polymerization of an organic siloxane polymer precursor in the emulsion droplets in the presence of PEG or toluene as a porogen, in which the siloxane polymer precursor is substituted with organic moieties. PEG or toluene is later washed out, eliminating the need of any high temperature burn-off. Examples of such particles are disclosed in U.S. Pat. Nos. 4,017,528, 6,686,035, and 7,223,473, and in WO2006039507.
While these prior art approaches provide superficially porous silica particles, there is a need to make both hybrid and non-hybrid superficially porous silica particles with a narrow particle size distribution, narrow pore size distribution, high specific surface area and a porous outer layer for faster separation, lower chromatography column pressure drop, and higher efficiency, together with stability at high pH and with good mechanical strength under chromatography conditions.